glsl_to_nir: update interface type properly

glsl_to_nir: update interface type properly

Since 76ba225184ed the member variable types were being redefined
but we assigned the old interface type to the variable.

In a following patch series we will use the types to check if we
are dealing with an interface instance when apply GLSL linking
rules.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3468>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3468>